1. Field of the Invention
Generally, the field of the present invention relates to wavelength stabilized laser diodes and laser diode modules.
2. Background
Lasers have enjoyed wide usage in different industries for years. Diode lasers in particular offer high electrical to optical efficiency, high output power, and reliable performance. For example, compact modules may be constructed that house a plurality of semiconductor diode lasers, either in the form of a bar of such diode lasers or singly and separate from each other. Laser light emitted from the diode lasers can then be fiber-coupled using miniature optics and the fiber-coupled light can in turn be used for various applications, such as directly pumping larger solid-state lasers or high power fiber amplifiers. However, the lasing wavelength and spectral width are current and temperature dependent, and are often too broad for the narrow absorption line of certain materials. The use of diode lasers to pump these materials requires methods to narrow the spectral linewidth of the device, and lock the output wavelength to a predetermined value.
Many methods of optical feedback have been developed to narrow and lock the lasing spectrum of laser diodes. By including frequency selective feedback techniques and suppressing the Fabry-Perot modes of the cleaved laser facets, the diode can be forced to laser at a designed wavelength, as opposed to lasing at the peak of the gain bandwidth on one of the closely spaced Fabry-Perot modes. These frequency selective feedback techniques can be fabricated or incorporated either inside the cavity, as is the case for distributed Bragg reflector (DBR) or distributed feedback (DFB) lasers, or external to the cavity, as is the case for external cavity lasers (ECLS) fabricated in a Littrow, Littman, volume Bragg gratings, or fiber Bragg gratings. In each of these cases, optical feedback is used to lock and narrow the spectrum of laser diodes.
Ideally, wavelength stabilized devices would have the same power and efficiency characteristics as unlocked devices, and would be able to operate over a wide temperature range. Unfortunately, the feedback mechanisms used in stabilize diode laser wavelength introduce optical loss, reducing the output power and operating efficiency of the device. Additionally, variations in the device temperature can cause a broadening and shift of the peak of the optical gain in the semiconductor device, causing the device to lase on the parasitic Fabry-Perot modes, as opposed to the design wavelength. Despite the need by industry of an external wavelength locking semiconductor diode laser apparatus that may operate substantially power penalty-free, no such device has been created.